As is well known, aircraft are susceptible to lightning strikes when in the vicinity of convective weather activity. It is therefore not uncommon for lightning to strike an aircraft while maneuvering on the ground or in the air when in proximity to weather conditions likely to generate lightning discharges. For this reason, a variety of on-board lightning protection systems are known and adopted by aircraft manufacturers, as is evident from U.S. Pat. Nos. 3,906,308 and 8,672,269 (the entire content of each being expressly incorporated hereinto by reference).
Fuel tanks for aircraft are typically installed in the aircraft's main wing structures. Fuel tanks must include a vent to the ambient atmospheric environment to prevent negative pressure from occurring within the tank when the quantity of on-board fuel decreases via engine consumption during flight operations. The vent also ensures that the fuel tank is not over-pressurized during re-fueling operations. Conventional wing fuel tank vents include a vent opening typically located on the underside skin of the wing in a zone of low probability of lightning strikes. A duct connects the vent opening to the wing mounted fuel tank to allow ambient external air to communication between the vent opening and the fuel tank.
Those regions of the aircraft having a relatively high probability for direct lightning stroke attachment are known as Zone 1 regions. Zone 1 regions are typically those aircraft regions on the extremities of the aircraft's wing, vertical tail and horizontal stabilizer surfaces, nose and engines. Zone 2 regions are those regions usually adjacent to Zone 1 regions that are secondary direct lightning zones subject to lightning swept strokes. For this reason, wing fuel tank vents are typically located outside of Zone 1 or 2 regions. Specifically, fuel wing vents are typically located in a Zone 3 region, i.e., a zone of low lightning strike probability, located near the aircraft's wing tip. Locating the vent openings outside of Zones 1 and 2 prevents a lightning stroke attachment from causing a streamer being initiated at the wing fuel vent opening at a time when the fuel vapor condition at the vent opening is critical. As a result of locating the vent openings outside of Zone1 and Zone 2 regions thereby avoids a local explosion that could propagate into the fuel tank causing a catastrophic explosion.
Tankage of fuel in the aircraft's fuselage causes several unique problems in terms of lightning protection since the aircraft's fuselage is entirely within a Zone 2 region of lightning streamer risk aft of the aircraft's Zone 1 nose region. As such, in order to provide safe venting of the fuselage mounted fuel tanks to a Zone 3 region as near as possible to the aircraft's main wing tips, various onerous systems must be employed. For example, in order to provide a vent outlet located in a Zone 3 region for on-board fuselage mounted fuel tanks, extensive piping, valves and fittings must be provided and routed between the fuselage mounted fuel tanks and their associated vent openings located on the main wings of the aircraft. As a result, added weight of such vent-related structures as well as lesser flow rates and pressures associated with the supply and transfer of fuel in the fuselage mounted fuel tanks will ensue.
What has been needed in the art, therefore, are lightning protection systems and methods for aircraft fuel tanks that are not mounted in the wing, e.g., within an aircraft's fuselage, that minimize if not eliminate entirely the disadvantages discussed above.